44 lines
1.1 KiB
C
44 lines
1.1 KiB
C
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#include "tommath_private.h"
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#ifdef MP_MONTGOMERY_CALC_NORMALIZATION_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis */
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/* SPDX-License-Identifier: Unlicense */
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/*
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* shifts with subtractions when the result is greater than b.
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*
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* The method is slightly modified to shift B unconditionally upto just under
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* the leading bit of b. This saves a lot of multiple precision shifting.
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*/
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mp_err mp_montgomery_calc_normalization(mp_int *a, const mp_int *b)
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{
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int x, bits;
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mp_err err;
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/* how many bits of last digit does b use */
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bits = mp_count_bits(b) % MP_DIGIT_BIT;
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if (b->used > 1) {
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if ((err = mp_2expt(a, ((b->used - 1) * MP_DIGIT_BIT) + bits - 1)) != MP_OKAY) {
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return err;
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}
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} else {
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mp_set(a, 1uL);
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bits = 1;
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}
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/* now compute C = A * B mod b */
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for (x = bits - 1; x < (int)MP_DIGIT_BIT; x++) {
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if ((err = mp_mul_2(a, a)) != MP_OKAY) {
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return err;
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}
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if (mp_cmp_mag(a, b) != MP_LT) {
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if ((err = s_mp_sub(a, b, a)) != MP_OKAY) {
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return err;
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}
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}
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}
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return MP_OKAY;
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}
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#endif
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